1. Field of the Invention
The present invention relates to a double-chuck mechanical pencil provided with a front chuck and a back chuck and, more particularly, to a double-chuck mechanical pencil provided with a front chuck for restraining the lead from retracting and a back chuck for extruding the lead, loosely fitted in a chuck ring.
2. Description of the Prior Art
Referring to FIG. 1 showing a known push-button double-chuck mechanical pencil provided with two chucks intended to reduce the residual lead, proposed in Japanese Patent Laid-open No. Sho 54-119435, a barrel cap a is attached to the front end of a barrel b, a back chuck d for extruding the lead by a predetermined length at a time is inserted in the barrel b and is biased backward by a spring c, and a front chuck e for gripping the lead is fitted in the cap a and is biased backward by a spring f. The back chuck d is advanced to feed the lead. After feeding the lead by a predetermined length, the back chuck d is unfastened and is further advanced by a predetermined distance necessary for pushing the front chuck e at the rear end so that the front chuck e opens. When the barrel cap a is removed from the barrel b in a state where no lead is held by the front chuck e, there is a possibility that the front chuck e slips off the barrel cap a. When the front chuck e is pushed to open by the back chuck d, the lead is caused to slip out of the front chuck e by its own weight.
Although various double-chuck mechanical pencils of this kind principally designed to reduce the residual lead have been proposed, these previously proposed double-chuck mechanical pencils are complex in construction and difficult to assemble and disassemble. Furthermore, in those previously proposed double-chuck mechanical pencils, if one of the two chucks is turned due to some cause while both the chucks are gripping the lead, the lead is twisted to break, a broken piece of the lead remains between the two chucks to obstruct the feed of the lead, and back end of the front portion of the lead and the front end of the back portion of the lead are unable to engage properly, making the extrusion of the following lead impossible.
For example, in a mechanical pencil having a coupling member screwed on a barrel, if the coupling member turns when separating the barrel from the coupling member to remove a broken piece of the lead, the back chuck is turned, the back portion of the lead gripped by the back chuck is twisted relative to the front portion of the lead gripped by the front chuck and, consequently, the lead is broken. If the user turns unintentionally the push button or the eraser holder connected through a lead tank to the back chuck, the back chuck is turned together with the lead tank, a back portion of the lead gripped by the back chuck is turned relative to a front portion of the lead gripped by the front chuck and, consequently, the lead is broken.